1. Field of the Invention
This invention relates to high strength lids for electronic circuit packages that avoid interference with circuit components within the package when a mechanical load is applied to the lid.
2. Description of the Related Art
Microelectronic circuits incorporating silicon or ceramic substrates often enclosed within a hermetically sealed package or housing that provides support and protection for the circuit. A number of electronic components and interconnecting metallizations are formed on a microcircuit board that is mounted to the base of the housing. The packages are hermetically sealed by a cover or lid over the sidewalls that surround the circuitry. The material most commonly used for both the housings and lids is Kovar.RTM. alloy manufactured by the General Electric Company, with a composition of 53% iron, 29% nickel and 18% cobalt; an optional gold or nickel plating may be added to prevent corrosion. The lids are welded or soldered to the upper surface of the package sidewalls, thereby hermetically sealing the packages.
Conventional lids are typically formed as simple flat plates, or in "hat-shaped" configurations in which the lid is elevated above the upper edges of the package sidewalls. Such walls and sealing methods are discussed in J. Holley, "Solder Sealing of Large Complex Hybrid microcircuits", ISHM National Symposium Proceedings (1969), pages 5-11. The lid must be able to withstand substantial loadings. Heavy loadings are encountered particularly prior to shipment, when the assembled circuit packages are tested to confirm the hermeticity of the seal. During this test the package is subjected to a significant pressure that can cause the lid to deflect downward toward the interior circuitry which it is designed to protect. If the downward deflection is excessive it can cause shorting and/or damage to the electrical components, and can also induce an excess stress in the weld seals that results in seal failure.
One way to strengthen the lid is to simply make it thicker. However, this increases both the package weight and its overall size. Another approach is to increase the height of the package sidewalls to maintain the necessary clearance above the electrical components and circuit traces under maximum lid depression. However, this adds undesirable volume to the package in excess of what is needed for the actual circuitry, and may violate height restrictions. For large packages, it has been necessary to provide posts inside the package to internally support the lid. While the posts provide sufficient strength, they reduce the amount of substrate area available for circuitry and, since circuit traces cannot be routed in the areas occupied by the posts, they increase both the difficulty of routing and the number of required routing layers. When the substrate is silicon, an even greater concern is that holes are typically drilled through the substrate to allow the post to contact the package base. This introduces the danger of cracking the fragile substrate, and therefore has a negative impact upon reliability.
An improved strengthened lid is disclosed in a pending patent application entitled "Domed Lid for Integrated Circuit Package", Ser. No. 465,851 filed Jan. 16, 1990 by Kovacs et al., and assigned to Hughes Aircraft Company, the assignee of the present invention. In this application the lid has a dome shape which adds to its ability to withstand external loads. However, the dome portion extends below the upper edge of the package's sidewalls, and reaches a minimum clearance above the base in the vicinity of the sidewalls. This can interfere with wire bonding near the wall and effectively reduces the amount of area available in this region.
For all of the above techniques in which the lid deforms under a load, the initial elastic deformation under relatively lighter loads occurs in a linear predictable fashion, and the lid recovers its original shape when the load is removed. However, for loads that go beyond this initial linear deformation region, the lid enters a plastic regime in which its deformation increases but at an unpredictable rate. If the load is removed after entering this regime, the lid will not recover its original shape; if the load is not removed in time it can stretch progressively to failure.